1. Technical Field
The embodiments herein generally relate to wireless communications systems, and, more particularly, to orthogonal-frequency-division-multiplexing (OFDM) systems.
2. Description of the Related Art
OFDM is a signal modulation technique used in wireless communications systems to transmit large amounts of digital data over a wireless channel and to reduce data interference caused by the wireless channel. In typical OFDM systems, a data signal is partitioned into multiple, smaller sub-signals, each of which is transmitted from a transmitter to a receiver over a different frequency and broadcast in all directions. The transmitted data signals typically reflect off objects in the environment, and may reach the receiver via different propagation paths.
Wireless communication systems that use OFDM may be categorized into linear, time-invariant (LTI) systems and linear, time-variant (LTV) systems. In LTI systems, channel conditions (for example, the strength of each propagation path) do not substantially vary with time, such as when a transmitter and receiver are stationary. Moreover, because channel conditions do not substantially vary, the receiver may use information pertaining to the channel conditions to easily extract desirable data from a received signal and to discard extraneous, undesirable data and/or effects imposed upon the signal by the wireless channel.
However, in LTV systems, substantially high Doppler conditions may exist (for example, where the distance between the transmitter and receiver varies at speeds of 200-300 km per hour). Accordingly, channel conditions (for example, the strength of each propagation path) may vary with time. Generally, these variations are caused by multiple signal reflections that are out of phase with respect to one another. Furthermore, because these rapid variations often occur faster than the rate at which the receiver can properly receive the data signals, the receiver may not be able to accurately account for these variations, which could result in interference between data on different frequency sub-carrier signals. This is often referred to as intercarrier interference (ICI).
Carrier-offset and the Doppler effect resulting from a dispersive time varying channel model represent two distinct sources of leakage in OFDM systems, which destroy the desired orthogonality among the corresponding subchannels. This gives rise to ICI whose effect must either be compensated or cancelled prior to detection. Accordingly, there remains a need for an equalization technique for OFDM receivers in the presence of ICI.